This invention relates generally to the pulping of wood chips and more particularly to a screening process for sizing and fractionating a flow of chips prior to pulping.
It has been recognized recently that the thickness dimension of wood chips plays a key role in the pulping process. Briefly, in the pulping process, a digester receives chips and then, through the use of chemicals, pressure, and elevated temperatures, breaks the wood down into its constituents, basically lignin and cellulose. The cellulose (wood fibers) is then further processed before making the pulp product. Of course, as will be well understood by those skilled in the art, in order to produce a uniform high yield pulp, the process must begin with a correctly sized and composed chip flow. Oversized (based on length and width) and overthick chips are not broken down well in the digester and at a subsequent screening stage larger unacceptable particles must be removed, thereby reducing pulp yield.
Undersized chips (referred to commonly as "fines") and other small undesired material in the chip flow such as sawdust and bark particles should also be removed from the flow prior to the digester. The undersized material can be overcooked in the digester and acts to weaken the overall pulp if allowed to remain. Thus, it is very important to generate and produce an optimum furnish (chip flow) for the digester in required quantities and at a low cost.
Chip screening systems are well known and the prior publication of E. Christensen appearing in the May 1976 TAPPI Journal, Vol. 59, No. 5 contains a disclosure of those in use today and in the past. For example, a typical gyratory screen is used in many applications for high particle separation efficiency for a given size of screen. The gyratory screen has less tendency to upend and remove elongated particles (pin chips) and there is less tendency to plug the screen openings with particles close to the opening size. There is also less tendency to abrade and break chips into smaller pieces.
Another typical screening means disclosed in the Christensen publication is the disk screen. It consists of a number of parallel rows of shafts on which properly spaced disks are mounted in such a way that the disks on one shaft are centered in the space between the disks on adjacent shafts. The spacing determines the size of chip that will fall through in any fraction and those that will stay atop the disk screen and be treated as "overs," that is, those that may be oversized and/or overthick. The disk screen, properly constructed, has been found to be especially useful in sorting chips according to the thickness dimension. The Christensen publication is incorporated herein by this reference in order to provide additional background information.
In generating the desired chip flow to feed the digester, one system approach using a disk screen and a gyratory screen in combination is disclosed in the Christensen reference. He suggests that, by first passing an incoming chip flow over a disk screen, the "overs" can be removed expeditiously from the flow and treated as an "overs" fraction. A fraction generated at the disk screen through a portion thereof will be "accepts," that is, chips falling within a predetermined size and thickness range and suitable for flow directly to the digester and another fraction will be the smaller material including pins, fines, sawdust and the like. If pin chips are suitable for the particular digester, they will be separated out and directed to the digester. Overs will be further processed to reduce their size and/or thickness to bring them into the predetermined acceptable range after which the original overs fraction will be a flow ready for the digester.
In most typical digesters, the ideal thickness range for softwood chips is from 2 mm to 10 mm and for hardwood chips from 2 mm to 8 mm. Chips falling within these ranges should come to the digester in a continuous flow and in the right quantity, depending upon the digester requirements.
Of course, chip sources can have a variety of characterizations depending upon their origin. Some sources may have a high percentage falling within the desired size and thickness range with few of the chip particles being either "overs" or "unders." Other sources may have relatively higher percentages of "overs," for example. Since chip sources vary, the screening system is designed with the variable nature of the source of chips in mind. The optimum screening system also, in addition to virtually eliminating overs from the flow, should retain all fiber suitable for digesting. This means pin chips should be used and losses of otherwise useable chips should be eliminated.
It has been found that by causing the incoming chip flow to first be processed by a primary screening station such as a two-deck gyratory screen resulting in three fractions and then causing at least a 30%-60% portion of the total flow which is the fraction not passing through the first deck or screening media to be screened at a second station by a disk screen resulting in two fractions (one being accepts and the other being overthick), not only is the chip flow efficiently processed but the capital cost is significantly reduced. It has also been found that by using two-station screening in the proper sequence, significant improvement in unders removel efficiency occurs. The resulting overthick fraction is passed through a thickness reducing means such as a chip slicer with the resulting flow then being considered "accepts."
If changes in either the process flows (weight/unit of time) or process pulping parameters occur, it had been a problem for prior systems to accommodate such changes without extensive changes and attendant high cost. The present invention allows process changes to be accommodated simply and at low cost without having more than a minimal effect on overall performance of the system.
Briefly, the present invention is practiced in one form by directing a flow of chips from a source typically having a percentage of overs ranging from 0-20% of the total and a similar percentage of unders to a primary screening station where the incoming flow is divided, based primarily on the length and width, into a fraction of overs, a fraction of accepts and a fraction of unders. The overs fraction is directed to a second screening station comprising a disk screen where a fraction of accepts is generated, based primarily on thickness as is a fraction of overs which is directed to a chip thickness reducing means. The accepts from the primary and second screening stations are combined with the flow from the chip reducing means to result in a flow having chips falling within the predetermined dimensional ranges.
Accordingly, from the foregoing, one object of the present invention is to reduce the oversize and/or overthick chips flowing to a digester.
Another object is to reduce the undersize material flowing to the digester.
Yet another object is to provide an efficient chip screening system at a lower capital cost.
A further object of the invention is to quickly and at low cost modify the structure of the system to accommodate process changes.
These and many other objects of the invention will become more apparent and better understood upon reviewing the description to follow in conjunction with the attached drawing.